Injection drug users (IDUs) are at high risk of blood-borne infections, including HIV, hepatitis B, and hepatitis C, through the sharing of needles, syringes, and other injection paraphernalia. Needle exchange programs (NEPs) have been introduced in both developed and developing countries in an effort to reduce sharing of injection equipment among IDUs and discarding of contaminated syringes (1,2). Most NEPs in the United States, in addition to providing sterile injection equipment, offer condoms, HIV risk reduction counseling, and passive referrals to drug treatment (3,4).
Initial research on NEPs tended to focus on concerns about their potential negative impact (1). Although some studies have reported continued risk behaviors among attenders of NEPs (5,6), numerous studies and reviews have concluded that NEPs did not result in increased numbers of IDUs or in increased frequency of injection (4–8), nor were NEPs associated with an increase in crime (9) or number of discarded needles (10). On the other hand, NEPs have been associated with reductions in citywide seroprevalence rates (11) and with reductions in HIV incidence (12–14). Use of NEPs has been associated with a reduction in high-risk behaviors such as borrowing and lending of used injection equipment among HIV-negative and HIV-positive IDUs (15–18). NEPs have had a critical role in averting HIV epidemics in the United Kingdom, Australia, and elsewhere (19,20).
In conjunction with sufficient treatment resources, NEPs can be an important bridge to drug treatment. A study in New Haven, Connecticut found that NEPs attracted not only persons who were exchanging needles but persons who were specifically seeking drug treatment (21). Studies in Baltimore have demonstrated that NEP attendance is associated with entry into both detoxification (22) and methadone maintenance (23). Data from Seattle indicate that NEP attenders were more likely to enter treatment and to remain in treatment than nonattenders (24).
Increasing the proportion of IDUs in drug treatment programs is an important public health priority. In the United States, although over 1 million persons were enrolled in substance abuse treatment on any given day in 1998 (25), approximately 40% to 45% of IDUs have never been enrolled in treatment (26,27). Well-designed and well-operated drug treatment programs, particularly methadone maintenance therapy, have been demonstrated repeatedly to reduce or eliminate drug use, to improve health, to increase employment, and to reduce criminal activity (7,28–35).
NEPs have been shown to attract more severely dependent IDUs who inject more frequently and engage in higher risk activities such as attending shooting galleries (16,36,37). NEPs thus have the potential to reach a subgroup of IDUs who are at extremely high risk of acquiring blood-borne infections. Nevertheless, the potential for NEPs to serve as a conduit to drug abuse treatment may be lessened if NEP attenders are less motivated to cease drug use or to change their drug use patterns relative to nonattenders.
A model of readiness for cessation of drug use has been developed, based on the model of behavioral change proposed by Prochaska and DiClemente (38,39). This model of behavioral change posits that individuals pass through a series of stages moderated by different processes before a behavioral change. The model comprises three prechange stages. In the precontemplation stage, the individual exhibits no intention to change the behavior. In the next stage, contemplation, the individual exhibits awareness that it would be desirable to change the behavior. In the third stage, preparation, the individual has decided to change the behavior but has not yet attempted to enact the change. This model has been applied to an increasingly wide range of behaviors, including exercise, contraceptive use, and dietary change (40).
Applications of the above “stages of change” model to cessation of drug use are relatively recent. The model has been used most widely in the evaluation of treatment programs to predict relapse and retention (41–44). Most dramatically, a study comprising 2265 persons in 11 US cities found that indicators of motivation, in particular readiness for drug treatment, predicted treatment engagement and retention (45). Further, motivation was a more significant predictor of these outcomes than was drug use. Among drug users not in treatment, a recent study found that the stage of change in readiness for cessation of drug use was predictive of treatment entry (46). A study of readiness to change drug use conducted among NEP attenders in Providence, Rhode Island found that 64% were ready to change their drug use but that more frequent NEP attendance did not have an impact on readiness to change drug use (47). This study did not include a comparison group of NEP nonattenders, however.
In this study, we used a validated scale measuring readiness for cessation of drug use to determine the relationship between NEP attendance and readiness for cessation of drug use. The study reported here permitted readiness scores for cessation of drug use to be compared across attenders and nonattenders of NEPs. Such data should help to guide future efforts that aim to engage a higher proportion of IDUs in drug treatment programs.
Study participants were 288 IDUs participating in the AIDS Link to Intravenous Experiences (ALIVE) study in Baltimore. Participants were interviewed between June 1998 and March 2000. The ALIVE study is a longitudinal investigation of the natural history of HIV-1 infection. Beginning in January 1988, 2921 IDUs were recruited over a 15-month period for semiannual follow-up. Eligible participants were aged 18 years or older at baseline and self-reported injection drug use within the prior 10 years. About 90% of participants were recruited through word of mouth and extensive community outreach. Details of sampling, recruitment, and data collection have been reported elsewhere (48,49). Participants provided voluntary informed consent. The study was approved by the Johns Hopkins School of Public Health Committee on Human Research.
For this study, ALIVE study participants were administered a “Readiness for Cessation of Drug Use Module” as part of the first follow-up visit occurring in the study period. Those who reported using drugs other than alcohol and marijuana during the past 30 days were eligible for the study. Participants were also asked about NEP attendance or nonattendance in the prior month. The study period was 21 months: recruitment of the original sample had occurred over 15 months, and an additional 6 months was allowed for participants whose visit schedule may have changed slightly over time. Eighty-eight percent of responses were obtained in the first 6 months of the study, however, and 95% were obtained in the first 12 months.
At the initial ALIVE study visit in 1988, eligible and consenting participants were assigned unique confidential identification numbers that were used on all subsequent data collection instruments. At baseline and semiannually thereafter, each participant underwent venipuncture for HIV-1 antibody assays and an interviewer-administered questionnaire. Questions included demographic information, medical history, drug use and drug injection practices, sexual activity, knowledge about HIV/AIDS, drug treatment history, and utilization of medical care. Beginning in August 1994, coinciding with the inauguration of the Baltimore NEP, a series of questions was included regarding acquisition of syringes and use of the NEP in the past month. Beginning in April 1998, a module designed to assess readiness for cessation of drug use was administered at the time of the routine follow-up visit.
Readiness for Cessation of Drug Use Module
The module assessing readiness to stop drug use was a modified version of a scale developed by Booth et al. (46), based on work by Simpson and Chatman (50) and Knight et al. (51). Subjects who had used drugs other than alcohol or marijuana in the past 30 days were asked to respond to a series of 28 Likert-scaled statements. Scores ranged from 1 (“Strongly disagree”) to 5 (“Strongly agree”). Stages were assigned based on earlier work by the authors validating the scale in the target population (52). Two domains of questions were used to assign readiness stage for drug use cessation. The first domain was recognition of problem drug use (“Your drug use is a problem for you,” “Your drug use is more trouble than it's worth,” and “Your drug use is under control” [reverse scored]). The second domain was intention to quit drug use (“You are ready to quit using drugs right now,” “You plan to quit using drugs in the next 30 days,” and “You plan to quit using drugs in the next six months”). Stages were assigned as follows.
Subjects were assigned to the precontemplation stage if they demonstrated no intention to quit drug use (a score of 3 or less in response to each of the “problem drug use” statements and a total score for these statements of 8 or less) or denial of problem drug use and no opinion on quitting drug use (a score of 3 in response to each of the “intention to quit” statements and a total score of 9 or less on the problem drug use statements).
Subjects were assigned to the preparation stage if they demonstrated acknowledgment of problem drug use and the intention to quit drug use (a score of 3 or more in response to each of the six statements and a total score of 12 or more for each group of statements).
Subjects were assigned to the contemplation stage if they demonstrated an inconsistent response to the determinant statements. This category was assigned if the criteria for other categories were not met. It included respondents who gave inconsistent opinions on whether drug use was a problem, those who acknowledged a problem but not a consistent intention to quit, and those who did not acknowledge a problem but expressed an intention to quit.
The Baltimore Needle Exchange Program
The Baltimore NEP was introduced in August 1994 by the Baltimore City Health Department. During the study period, the Baltimore NEP consisted of two mobile vans serving seven locations. The program operates as a one-for-one syringe exchange with no upper limit on the number of syringes that can be exchanged at a given time. The NEP distributes harm reduction kits, condoms, and HIV prevention brochures. It offered free HIV testing with counseling and referrals to 90 subsidized methadone maintenance slots during the period under study. State legislation exempts participants and staff from prosecution under drug paraphernalia laws in Baltimore City. Through December 2000, over 12,000 Baltimore IDUs were registered as NEP attenders, among whom 630,000 syringes were exchanged annually.
Frequency distributions were generated by stage of change and NEP attendance for all variables of interest. To compare the three stages of change, univariate logistic regression under a proportional odds model was used. The proportional odds model takes into account the ordinal nature of the response variable (i.e., stage of change). This model assumes that the association with the explanatory variables is independent of the cutoff point of the cumulative logit, (i.e., that there is a common parameter, β, regardless of where the ordinal response variable is divided) (53). Readiness for cessation of drug use was therefore dichotomized, with those in the preparation stage classified as “ready.” Univariate logistic regression was used to compare those were ready and those who were not with respect to variables of interest and to compare NEP attenders and nonattenders with respect to the same variables.
Finally, multiple logistic regression models were fit to identify independent predictors of readiness for cessation of drug use. Variables that were significant at the 10% level were considered for inclusion in the multivariate model. All two-way interactions were explored.
Respondents were 77% male and 93% African-American compared with 81% male and 89% African-American in the baseline ALIVE study sample (48). Median age in the analysis sample was 43 years (interquartile range: 39–48 years), consistent with the median age of 34 years reported about a decade earlier at the baseline ALIVE study visit. The study sample was similar to the original cohort in age, sex, and educational level (p > .05); however, the study sample was more likely to include persons who were actively using substances other than alcohol and marijuana (p < .001), because this was a criterion for eligibility.
Seventeen percent of the sample had been enrolled in drug treatment in the past 6 months; this was consistent with earlier reports (23). In both the original and analysis samples, 24% of participants were HIV-positive at baseline; 44% of the analysis sample was HIV-positive at follow-up.
Table 1 presents the sociodemographic characteristics of the sample by readiness for cessation of drug use. Overall, 30% (n = 87) were classified as ready to cease drug use (i.e., in the preparation stage [recognition of both drug use problems and the need for help]). Of the 70% classified as not ready, 22% (n = 44) were in the precontemplation stage (no intention of stopping drug use) and 78% (n = 157) were in the contemplation stage (data not shown). Readiness for cessation of drug use was marginally associated only with age; persons aged 45 years and older were slightly less likely to be ready for cessation of their drug use compared with younger participants (p = .08).
Table 2 presents drug use characteristics by readiness for cessation of drug use. Readiness for cessation of drug use was significantly associated with increased use of speedball injection (i.e., injection of a combination of heroin and cocaine [OR = 2.3]). Thirty-five percent of those who had injected speedball in the past 6 months were ready for cessation compared with 19% of those who had not. Heroin injection was also associated with readiness for cessation of drug use (OR = 2.0); 34% of those who had injected heroin in the past 6 months were ready for cessation compared with 21% of those who had not. Neither duration of injection career nor frequency of injection was associated with readiness for cessation of drug use. NEP attendance was not associated with readiness for cessation of drug use (OR = 0.9).
As seen in Table 3, neither HIV status nor other injection-associated medical problems were associated with readiness for cessation of drug use. Use of an emergency room in the past 6 months was associated with readiness for drug use cessation (OR = 1.7), however. Seventeen percent of the sample had been enrolled in drug treatment in the past 6 months. Not surprisingly, these individuals were more than twice as likely to be ready for cessation of drug use (OR = 2.4). Those who had been enrolled in a detoxification program in the past 6 months were three times more likely to be ready for cessation of drug use (OR = 3.1). Having been in treatment in the past year was also significantly associated with readiness for cessation of drug use.
Because we did not find a difference between NEP attenders and nonattenders in terms of readiness for cessation of drug use, we compared these groups to determine whether they differed according to demographics or drug use behaviors (data not shown). Compared with nonattenders, NEP attenders were significantly more likely to inject daily (OR = 2.8), to inject speedball (OR = 2.5), and to have been homeless in the previous 6 months (OR = 2.3).
Table 4 shows the results of several multivariate models and the effect of possible confounders. Model 1 includes all variables that were associated in univariate analysis with either readiness for cessation of drug use or NEP attendance at the p < .10 level. The exception was drug treatment history; several different measures were associated with readiness for drug use cessation. Inclusion of more than one of these measures effectively nullified the contribution of any. Therefore only a single measure, drug treatment in the past 6 months, was included. In Model 1, speedball injection in the past 6 months (OR = 2.4) and having been in drug treatment in the past 6 months (OR = 2.3) were significantly associated with readiness for cessation of drug use. Injection of heroin in the past 6 months was marginally significant (OR = 1.8). Model 2 presents the most parsimonious model. ORs for the two significant associations are slightly higher than in Model 1; however, inclusion or exclusion of other variables had little effect on the ORs or CIs. No two-way interactions were observed that significantly improved model fit.
Among our sample of active IDUs in Baltimore, 30% were classified as ready for drug use cessation, having acknowledged that their drug use was a problem and expressed the intention to quit. Significantly, we found no difference in readiness for drug use cessation among NEP attenders versus nonattenders, despite the fact that we and others have shown NEP attenders to report behaviors that are consistent with more severe drug dependence (16,36,37).
Far from being a barrier to treatment entry, the more severe drug dependence of NEP attenders may increase their likelihood of treatment entry if given ample opportunity, because treatment seeking has been consistently associated with greater drug dependence (46,54–56). Further, our study confirms the strong association between treatment seeking and prior treatment experience, indicating that persons who have become engaged in the treatment process are more likely to return to treatment after a relapse to drug use.
These data support earlier work from our group showing that IDUs referred to methadone maintenance through NEPs had similar retention rates and decreases in addiction severity compared with persons referred through other sources (57). In Amsterdam, NEP attendance was not a significant factor affecting cessation of injection drug use when changes in methadone dosage were taken into account (58), indicating that NEP use did not interfere with drug abuse treatment among persons who continued to inject.
Taken with our previous research showing that NEP attendance is associated with entry into drug treatment, the present findings suggest that by expanding the availability of NEPs in the United States and establishing formal referral linkages between NEPs and drug treatment programs, the proportion of IDUs in treatment could be dramatically increased. As of December 2000, there were 134 NEP programs operating in the United States compared with some 2000 in Australia and several hundred in Great Britain (2). In the United States, many of these programs operate illicitly or semi-illicitly. According to a 1996 survey of 81 NEPs in the United States, 50% of NEPs had formal relationships with drug treatment programs but illegal NEPs were significantly less likely to have such relationships (3).
In conjunction with expanding NEP programs, it is important to ensure that an adequate number of treatment slots exist. Earlier studies indicated that although NEP attenders were more likely to enter methadone maintenance or detoxification than nonattenders, there was a decline in the association over time corresponding with saturation of treatment services (22,23).
Similarly, an active drug treatment referral system associated with an NEP in New Haven, Connecticut was compromised when the city welfare department introduced a managed care initiative that excluded the NEP (21). Our findings underscore the need for sustained linkages between NEPs and existing programs that aim to attract and retain drug users in treatment.
Several study limitations should be acknowledged. The generalizability of this study is limited. Originally a large, community-based, out-of-treatment cohort, the ALIVE study participants have now been followed for 10 years and thus represent survivors. Lower risk IDUs have been more likely to drop out of the study (59). The study findings may not be generalizable to younger IDUs and to IDUs who began to inject recently. The age of IDUs in this cohort is not atypical among inner-city largely African-American IDU populations, however.
Our study supports the need to integrate NEPs and drug treatment programs to lessen the burden of injection drug use as well as to contain epidemics of HIV and other blood-borne diseases. Because non-NEP attenders represent an out-of-treatment population that is hard to reach, our findings provide further evidence that NEPs can play a significant role in attracting IDUs who may be ready to avail themselves of drug abuse treatment. There is a pressing need to support and maintain formal arrangements between NEPs and drug treatment programs and to ensure that an adequate number of treatment slots are maintained and made financially available to IDUs.
The authors thank Peter Beilenson, the staff and participants of the ALIVE study, and the Baltimore Needle Exchange Program.
1. Paone D, Des Jarlais DC, Gangloff R, et al. Syringe exchange: HIV prevention, key findings, and future directions. Int J Addict 1995; 30:1647–83.
2. Bastos FI, Strathdee SA. Evaluating effectiveness of syringe exchange programmes: current issues and future prospects. Soc Sci Med 2000; 51:1771–82.
3. Paone D, Clark J, Shi Q, et al. Syringe exchange in the United States, 1996: a national profile. Am J Public Health 1999; 89:43–6.
4. Lurie P, Reingold AL, Bowser B, et al. The public health impact of needle exchange programs in the United States and abroad. Summary, conclusions and recommendations.
San Francisco: Berkeley Institute for Health Policy Studies, School of Public Health, University of California, 1993.
5. Hahn JA, Page-Shafer K, Lum PJ, et al. Hepatitis C virus infection and needle exchange use among young injection drug users in San Francisco. Hepatology 2001; 34:180–7.
6. Valenciano M, Emmanuelli J, Lert F. Unsafe injecting practices among attendees of syringe exchange programmes in France. Addiction 2001; 96:597–606.
7. National Institutes of Health/National Consensus Development Panel on Effective Medical Treatment of Opiate Addiction. Effective medical treatment of opiate addiction. JAMA 1998; 280:1936–43.
8. Normand J, Vlahov D, Moses LE. Preventing HIV transmission: the role of sterile needles and bleach.
Washington, DC: National Academy Press, 1995.
9. Marx MA, Crape B, Brookmeyer RS, et al. Trends in crime and the introduction of a needle exchange program. Am J Public Health 2000; 90:1933–6.
10. Doherty MC, Junge B, Rathouz P, et al. The effect of a needle exchange program on numbers of discarded needles: a 2-year follow-up. Am J Public Health 2000; 90:936–9.
11. Hurley SF, Jolley DJ, Kaldor JM. Effectiveness of needle-exchange programmes for prevention of HIV infection. Lancet 1997; 349:1797–800.
12. Des Jarlais DC, Marmor M, Paone D, et al. HIV incidence among injecting drug users in New York City syringe-exchange programmes. Lancet 1996; 348:987–91.
13. Kaplan EH, Heimer R. HIV incidence among needle exchange participants: estimates from syringe tracking and testing data. J Acquir Immune Defic Syndr Hum Retrovirol 1994; 7:182–9.
14. van Ameijden EJ, van den Hoek AR, Coutinho RA. Injecting risk behavior among drug users in Amsterdam, 1986 to 1992, and its relationship to AIDS prevention programs. Am J Public Health 1994; 84:275–81.
15. Hagan H, Des Jarlais DC, Purchase D, et al. An interview study of participants in the Tacoma, Washington, syringe exchange. Addiction 1993; 88:1691–7.
16. Vlahov D, Junge B, Brookmeyer R, et al. Reductions in high-risk drug use behaviors among participants in the Baltimore Needle Exchange Program. J Acquir Immune Defic Syndr Hum Retrovirol 1997; 16:400–6.
17. Watters JK, Estilo MJ, Clark GL, et al. Syringe and needle exchange as HIV/AIDS prevention for injection drug users. JAMA 1994; 271:115–20.
18. Vertefeuille J, Marx MA, Tun W, et al. Decline in self-reported high risk injection-related behaviors among HIV seropositive participants in the Baltimore Needle Exchange Program. AIDS Behav 2000; 4:381–7.
19. Stimson GV. AIDS and injecting drug use in the United Kingdom, 1987–1993: the policy response and the prevention of the epidemic. Soc Sci Med 1995; 41:699–716.
20. Des Jarlais DC, Hagan H, Friedman SR, et al. Maintaining low HIV seroprevalence in populations of injecting drug users. JAMA 1995; 274:1226–31.
21. Heimer R. Can syringe exchange serve as a conduit to substance abuse treatment? J Subst Abuse Treat 1998; 15:183–91.
22. Strathdee SA, Celentano DD, Shah N, et al. Needle-exchange attendance and health care utilization promote entry into detoxification. J Urban Health 1999; 76:448–60.
23. Shah NG, Celentano DD, Vlahov D, et al. Correlates of enrollment in methadone maintenance treatment programs differ by HIV-serostatus. AIDS 2000; 14:2035–43.
24. Hagan H, McGough JP, Thiede H, et al. Reduced injection frequency and increased entry and retention in drug treatment associated with needle-exchange participation in Seattle drug injectors. J Subst Abuse Treat 2000; 19:247–52.
25. Substance Abuse and Mental Health Services Administration. Uniform facility data set (UFDS): 1998.
Drug and Alcohol Services Information System Series: S-10. DHHS Publication (SMA) 00-3463. Rockville, MD: Substance Abuse and Mental Health Services Administration, 2000.
26. Liebman J, Knezek LD, Coughey K, et al. Injection drug users, drug treatment, and HIV risk behavior. In Brown BS, Beschner GM, eds. Handbook on risk of AIDS: injection drug users and sexual partners.
Westport, CT: Greenwood Press, 1993:355–73.
27. Jackson JF, Rotkiewicz LG, Quinones MA, et al. A coupon program: drug treatment and AIDS education. Int J Addict 1989; 24:1035–51.
28. Kreek MJ. Rationale for maintenance pharmacotherapy of opiate dependence. Research Publications: Association for Research in Nervous and Mental Disease 1992; 70:205–30.
29. National Institute on Drug Abuse. Methadone maintenance treatment: translating research into policy.
Washington, DC: NIDA, 1995.
30. Office of National Drug Control Policy. Treatment protocol effectiveness. A white paper of the Office of National Drug Control Policy. J Subst Abuse Treat 1996; 13:295–319.
31. McLellan AT, O'Brien CP, Metzger D, et al. How effective is substance abuse treatment—compared to what? Research Publications: Association for Research in Nervous and Mental Disease 1992; 70:231–52.
32. White JM, Dyer KR, Ali RL, et al. Injecting behavior and risky needle use amongst methadone maintenance clients. Drug Alcohol Depend 1994; 34:113–9.
33. Caplehorn JRM, Ross MW. Methadone maintenance and the likelihood of risky needle-sharing. Int J Addict 1995; 30:685–98.
34. Wells EA, Calsyn DA, Clark LL, et al. Retention in methadone maintenance is associated with reductions in different HIV risk behaviors for women and men. Am J Drug Alcohol Abuse 1996; 22:509–21.
35. MacGowan RJ, Brackbill RM, Rugg DL, et al. Sex, drugs and HIV counseling and testing: a prospective study of behavior-change among methadone-maintenance clients in New England. AIDS 1997; 11:229–35.
36. Bruneau J, Lamothe F, Franco E, et al. High rates of HIV infection among injection drug users participating in needle exchange programs in Montreal: results of a cohort study. Am J Epidemiol 1997; 146:994–1002.
37. Schechter MT, Strathdee SA, Cornelisse PG, et al. Do needle exchange programmes increase the spread of HIV among injection drug users? An investigation of the Vancouver outbreak. AIDS 1999; 13:F45–51.
38. DiClemente CC, Prochaska JO. Self-change and therapy change of smoking behavior: a comparison of processes of change in cessation and maintenance. Addict Behav 1982; 7:133–42.
39. Prochaska JO, DiClemente CC. Stages and processes of self-change of smoking: toward an integrative model of change. J Consult Clin Psychol 1983; 51:390–5.
40. Prochaska JO, Velicer WF, Rossi JS, et al. Stages of change and decisional balance for 12 problem behaviors. Health Psychol 1994; 13:39–46.
41. Belding MA, Iguchi MY, Lamb RJ, et al. Stages and processes of change among polydrug users in methadone maintenance treatment. Drug Alcohol Depend 1995; 39:45–53.
42. Belding MA, Iguchi MY, Lamb RJ. Stages and processes of change as predictors of drug use among methadone maintenance patients. Exp Clin Psychopharmacol 1997; 5:65–73.
43. De Leon G, Melnick G, Kressel D, et al. Circumstances, motivation, readiness, and suitability (the CMRS scales): predicting retention in therapeutic community treatment. Am J Drug Alcohol Abuse 1994; 20:495–515.
44. Melnick G, De Leon G, Hawke J, et al. Motivation and readiness for therapeutic community treatment among adolescents and adult substance abusers. Am J Drug Alcohol Abuse 1997; 23:485–506.
45. Joe GW, Simpson DD, Broome KM. Effects of readiness for drug abuse treatment on client retention and assessment of process. Addiction 1998; 93:1177–90.
46. Booth RE, Kwiatkowski C, Iguchi MY, et al. Facilitating treatment entry among out-of-treatment injection drug users. Public Health Rep 1998; 113(Suppl 1):116–28.
47. Bluthenthal RN, Longshore D, Stein M, et al. Factors associated with readiness to change drug use among needle exchange users. Drug Alcohol Depend 2001; 62:225–30.
48. Vlahov D, Anthony JC, Muñoz A, et al. The ALIVE study: a longitudinal study of HIV-1 infection in intravenous drug users: a description of methods. J Drug Issues 1991; 21:759–76.
49. Anthony JC, Vlahov D, Celentano DC, et al. Self-report interview data for a study of HIV-1 infection among intravenous drug users: description of methods and preliminary evidence on validity. J Drug Issues 1991; 21:739–57.
50. Simpson DD, Chatman LR. TCU/DATAR forms manual: improving drug abuse treatment, assessment, and research (DATAR) project.
Fort Worth, TX: Institute of Behavioral Research, 1995.
51. Knight K, Holcom M, Simpson DD. TCU psychosocial functioning and motivation scales: manual on psychometric properties.
Fort Worth, TX: Institute of Behavioral Research, 1994.
52. Henderson LA. Readiness to stop drug use: a stage-of-change model in a population of current and former injection drug users.
University Microfilm International, 2000.
53. Stokes ME, Davis CS, Koch GG. Categorical data analysis using the SAS system.
Cary, NC: SAS Institute, 1995.
54. Sibthorpe B, Fleming D, Tesselaar H, et al. The response of injection drug users to free treatment on demand: implications for HIV control. Am J Drug Alcohol Abuse 1996; 22:203–13.
55. Schütz CG, Rapiti E, Vlahov D, et al. Suspected determinants of enrollment into detoxification and methadone maintenance treatment among injecting drug users. Drug Alcohol Depend 1994; 36:129–38.
56. Zule WA, Desmond DP, Vogtsberger KN. Injecting drug users who want treatment. Am J Drug Alcohol Abuse 1997; 23:507–22.
57. Brooner R, Kidorf M, King V, et al. Drug abuse treatment success among needle exchange participants. Public Health Rep 1998; 113(Suppl 1):129–39.
58. Langendam MW, van Brussel GH, Coutinho RA, et al. Methadone maintenance and cessation of injecting drug use: results from the Amsterdam Cohort Study. Addiction 2000; 95:591–600.
59. Nelson KE, Vlahov D, Solomon L, et al. Temporal trends of incident human immunodeficiency virus infection in a cohort of injecting drug users in Baltimore, MD. Arch Intern Med 1995; 155:1305–11.